Forces in Fluids:
PRESSURE
Chapter 12。１
What is Pressure?
• The word pressure is related to another
word. Do you know what it is?
• PRESSure
• Example – When you lean against a wall
you exert pressure against it.
Force and Pressure
• Why don’t you sink into snow when you
are wearing snow shoes?
• Why does it hurt so much when someone
with high heels steps on your foot?
• The amount of pressure is determined by
the area over which the weight (force) is
distributed.
Calculating Pressure
• Formula:
Pressure =
Force
__________
F
Area
P
A
Calculating Pressure
• Pressure (Defined):
• The force on a surface divided by the total
area over which it is exerted.
Calculating Pressure
• How is force measured?
• How is area measured?
• The unit for pressure is: pascal (Pa)
1 Pa =
1N
__________
1m2
TIME OUT! Let’s Review Area…
• Area is a measure of a surface.
• Area is found by multiplying the length
times the width.
• Example: Compute the area of a
rectangle that is 2 meters wide by 3
meters long.
• Answer = 6 m2
Calculating Pressure
• A lower pressure can be produced by…
• … increasing the area a force acts on.
• Or you can…
• … produce higher pressure by decreasing
the area a force acts upon.
Let’s try an example together
• A solid block of wood has a square base
1m x 1m and weighs 5000 N. How much
pressure does it exert on the floor
underneath?
• Pressure = Force/Area
• Pressure = 5000N / (1m x 1m)
• Pressure = 5000N/ 1m2
• Pressure = 5000 N/m2 or 5000 Pa
Try this one on your own…
• A baseball strikes a catcher’s mitt with a
force of 200N. If the area the ball strikes
is 0.04 m2, what is the pressure on the
mitt?
• Pressure = Force/Area
• Pressure = 200N/0.04 m2
• Pressure = 5000 Pa
Fluid Pressure
• A fluid can be either a liquid or a gas.
• A fluid is a substance that can easily flow
and is able to change shape.
• Examples?
Fluid Pressure
• All of the forces exerted by the individual
molecules…
• … in a fluid add together to make up the
pressure exerted by the fluid.
Fluid Pressure All Around
• The pressure exerted by air is…
• …referred to as air pressure or
atmospheric pressure.
• How does atmospheric pressure relate to
weather?
• The atmospheric pressure determines
whether it is a sunny day or rainy day.
Fluid Pressure All Around
• Air exerts pressure because…
• … it has mass.
• Each cubic meter of air around you has a mass
of about 1 kilogram.
Fluid Pressure All Around
• Average air pressure at sea level is…
• … 10.13 N/cm2.
• Given that the surface area of your hand is
about 100 cm2, the total pressure on your hand
is about 1000 N or 225 $%@*!
• How can your hand possibly support the weight
of the atmosphere when you don’t feel a thing?
Balanced Pressures
• In a fluid that is not moving…
• … pressure at a given point is exerted
equally in all directions.
Balanced Pressures
• Pressure inside your body…
• …balances the air pressure outside your
body.
Balanced Pressures
• This pressure comes from…
• … fluids inside your body.
• Some parts of your body contain gases
(lungs and sinus) and others contain
liquids (blood and cells).
Variations in Fluid Pressure
Pressure and Elevation
Air pressure decreases as elevation
increases.
• This is because there is less air/weight on
top of you.
• Why is the cabin of an airplane pressurized?
Variations in Fluid Pressure
Pressure and Elevation
• Your ears “pop” (as you go up in altitude)
because…
• … for a moment, the air pressure behind
your eardrums is greater than it is outside.
Variations in Fluid Pressure
Pressure and Depth
Fluid pressure depends on depth.
Variations in Fluid Pressure
Pressure and Depth
• Water pressure…
• … increases as depth increases.
• Why are submarines able to submerge so far
underwater?
Variations in Fluid Pressure
Pressure and Depth
• At greater depths, there
are more fluid molecules,
greater mass, and thus
more pressure.
• In the deepest parts of
the oceans, pressure can
be 1000 times that of sea
level!
Barometer
• An instrument called a barometer
is used to measure atmospheric
pressure.
• In this type of barometer, the
height of the liquid column
increases as atmospheric
pressure increases.
• High atmospheric pressure
indicates sunny weather.
• Low pressure means rainy
weather.
Pressure Information and
Demonstrations
• Two common fluid pressures are air and
water.
• Air pressure decreases as you go to higher
elevations.
• Water pressure increases the deeper you
go.
• Fluids will move from areas of high
pressure to low pressure.
Punch This!!!
• Why is it helpful to poke a 2nd hole in a
juice can?
• It reduced pressure on the one hole and
allows the juice to flow out faster.
• The second hole allows air to flow into
one hole and out the other.
Silly Straws
• Why does a straw work?
• When you suck on the straw, you are removing
the air from the straw (removing air pressure).
Fluid flows from high pressure to low pressure
(up the straw).
• Why does placing a finger over the top of the
•
straw keep the water from falling out?
High pressure inside the room is holding the
juice up in the straw.
Bed of Nails
• Why did the balloon pop when only one
nail was applied and not when many nails
were applied?
• The more surface area you have the less
pressure you will have.
– With the large bed of nails you have more
surface area spreading the pressure out
– With the single nail, the surface area was
smaller creating more pressure.
Holes in Three
• What did you observe about the flow of water
•
from each of the three holes?
The water out of the bottom hole came out
pretty far, the water out the top hole did not
come out very far at all.
• Why does this occur?
• The bottom had the weight (Force) of all the
water above it pressing down on it so that it
would come out with more pressure
Vacuum Chamber
• What happened to the object when all the
air was taken out of the container?
• The marshmallow man expanded to about
twice his size.
• Explain why this happened?
• Less air pressure allowed him to expand.
Cards & Water
• Why did the water stay in the glass?
• Higher pressure outside, lower pressure
inside the glass.
• Explain why this happened.